Methodologies for the Measurement of Water Flow in Grapevines

Chapter

Abstract

Many methods are available to calculate mass flow of water in the transpiration stream by using heat as a tracer. Measurements can be taken in herbaceous and woody tissues, and in any conductive organ including roots. Depending of the method, measurements are taken either in the part of the conductive organ where the sensors are located, or in the whole perimeter of the conductive organ. Some methods integrate the sap flow in the whole sapwood, while others give information to calculate sap flow at different depths below the cambium. Calibration is convenient in all cases, being compulsory for the invasive methods, since probe insertion alters the xylem characteristics. This chapter describes two main groups of methods, invasive and non-invasive. For each method, brief theoretical information and practical considerations are given. All the information help the users to choose the most suitable method for his/her own purposes.

Abbreviations

HPV

Heat pulse velocity

HP

Heat pulse method

CHP

Compensation heat pulse

HD

Heat dissipation

HFD

Heat field deformation

HRM

Heat ratio method

SHB

Stem heat balance

THB

Trunk heat balance

5.1 Introduction

Grapevines as well as other vines present several morphological characteristics that suggest a high water use compared to other broadleaved species. Large xylem vessels allowing for high flow rates (Zimmermann 1983), rapid shoot and leaf development even under dry conditions, and an extended root system capable of exploring large soil volumes for water (Richards 1983), are some of the typical characteristics to all Vitis species. Calculations of seasonal water use are obtained from changes in soil water content over time. However, their low time resolution as well as sampling problems due to the extensive root system results in guidelines rather than precise numbers which would be necessary for describing and modeling the reaction of a plant to environmental changes. Moreover, the contribution of different vegetation types like green cover crops or the vines themselves cannot be separated.

During the last decade, sap flow measurements have become increasingly popular, especially after several systems have been marketed. Sap flow methods are easily automated, allowing continuous records of plant water use with high time resolution. Understanding of the theoretical basis of the techniques of sap flow measurement is needed in order to select the most appropriate methods for each application and to avoid and decrease potential errors. Previous reviews (Smith and Allen 1996), describe the most important methods for measuring sap flow in plant stems. This chapter, update previous knowledge of sap flow techniques and describes several aspects (basis, technical specifications, advantages and problems) of sap flow systems used in grapevines.

5.2 Invasive Methodologies

5.2.1 Granier Heat Dissipation Technique (hd)

The method of Granier based on the heated probe technology, developed by Vieweg and Ziegler (1960), independently also by Ittner (1968), Balek and Pavlik (1977) and better quantified by Granier (1985, 1987). It is based on the detection of convective heat transport (heat carried with the sap stream). Two cylindrical probes with a diameter of 2 mm and length of 20 mm are inserted radially into the stem in a vertical distance of 100–150 mm. The upper probe is heated constantly and the temperature difference between the two probes is measured (Fig. 5.1). Under no flow conditions, the temperature in a zone around the heater is increased up to the point where the heat conduction through the wood is in equilibrium with the heat energy supplied by the heater. Then, the temperature differential is at maximum, decreasing rapidly when the heat energy around the upper probe is transported away with the xylem stream. Granier (1985) developed mean sap flow velocity and measured temperature differences that are valid for a number of species:
$${\textrm{U}} = 119*10^{ - 6} {\textrm{ K 1}}{\textrm{.231 (m}}^{\textrm{3}} {\textrm{m}}^{ - {\textrm{2}}} {\textrm{s}}^{ - 1} )$$
where U = sap flux density
  • \({\textrm{K}} = (\Delta {\textrm{T}}_{{\textrm{max}}} - \Delta {\textrm{T}})*\Delta {\textrm{T}}^{ - 1} \) (dimensionless)

  • \(\Delta {\textrm{T}}_{{\textrm{max}}} = \) temperature difference under no flow conditions

  • ΔT = measured temperature difference.

Fig. 5.1

Granier technique (scheme and picture from Dynamax Inc.)

To calculate mass flow, sap flux density (U), is then multiplied by the conducting sapwood area at the height of the heated probe.

Braun and Schmid (1999b) developed some adaptation of this system for sap flow measurements in grapevine, modifying the probe length, and demonstrated that grapevine stems do not develop heartwood and that the whole cross section area excluding the bark can be regarded as sapwood.

5.2.1.1 Practical Considerations

One drawback of the Granier heat dissipation method (HD) is that requires calibration during a period of zero sap flow (Granier 1985). A period of zero sap flow is usually assumed to take place at night-time, and a maximum temperature difference (ΔTmax) between a heated and a reference unheated probe is recorder predawn. However, sap flow does not necessarily cease in branches and roots at night; thus, estimates of sap flux density based on such principle may be incorrect. Sap may continue to flow during the first hours in the night because of water movement for new growth, water redistribution in roots (Sakuratani 1981) and refilling of stems tissue after prolonged drought (Escalona et al. 2002). Far from being an exception, night-time sap flow is widespread among species and range from 5 to 30% of daily water loss. The zero flux assumption is not restricted to HD, but also applies to the heat balance methods of Cermak et al. (1972) and Sakuratani (1981) and the Tmax heat pulse method of Green et al. (2003). These methods are described later. Using the Granier system, Tatarinov et al. (2005) have shown that for needle type probes, the ratio of the temperatures differences between the heater and the unheated probe inserted in the trunk (ΔT), depends of the heat conductivity of the wood. Because ΔTmax depends on wood heat conductivity (0.15–0.40 Wm−1K−1), changes in wood water content are expected to produce significant variations in ΔTmax. The scenario is further complicated because Granier formula is sensitive to the choice of ΔTmax, so that undetected small amounts of sap flow during the night may lead to large errors in sap flux density computations. In order to resolve this problem, Regalado and Ritter (2007), propose a relationship between ETP obtained by Penman Monteith equation and ΔTmax for the estimation of zero flow considering ΔT and introducing a constant in the formula.

5.2.2 Heat Pulse Systems

Heat pulse method permit measurements of sap flow rate by determining the velocity of a short pulse of heat carried by the moving sap stream. Heating and sensor probes must be installed by drilling holes into sap wood, so this method is suitable only for use on woody stems like vines.

Heat pulse methods date back some 70 years to the work of Huber (1932), who first conceived the idea of using heat as a tracer of sap flow. Almost 25 years later, Marshall (1958) developed a theoretical framework for heat pulse based on a set of analytical solutions.

Swanson (1962) was one of the first to use Marshall’s analytical solution in his analysis of the compensation heat pulse method (CHP method). The velocity of sap ascending a stem is determined by compensation of the measured velocity of a heat pulse for the dissipation of heat by conduction through the matrix of wood fibres, water and gas within the stem (Swanson 1994). Two temperature sensors are placed asymmetrically on either side of a line of heat source. Immediately after release of a pulse of heat of 1–2 s duration, temperature increases more in the upstream sensor than at the downstream sensor because of conduction. Nevertheless, the heat carried by the moving sap quickly warms the downstream sensor, so the temperature of the two sensors equalizes after some time (te) approximately 60 s. This is the time required for convection in the moving sap stream to move the peak of the heat pulse from the heater to the point midway between the two temperature sensors, so te decreases as sap velocity increases. The velocity of the heat pulse (vh) is this given by the following equation (Swanson and Whitfield 1981)
$${\textrm{V}}_{\textrm{h}} = ({\textrm{x}}_{\textrm{d}} - {\textrm{x}}_{\textrm{u}} )/2{\textrm{t}}_{\textrm{e}}$$

Where the (te) is the time (s) delay for the temperatures at points xu and xd to become equal. And xd and xu are the distances (cm) between the heater and the upstream and downstream sensors, respectively.

Because sap velocities in woody stems normally vary with radial depth, sensors are usually implanted at several depths below the cambium of the stem, so that the radial profile of sap flux density across the sapwood can be determined. Mass flow rates of sap through the stem (Fm) are then calculated from the integral of the sap flux profile over the cross-sectional area of the sapwood. For species with sapwood considered to be thermally homogeneous, like vines, the heat pulse technique can be used to measure transpiration almost without calibration. Nevertheless, validation of this technique and derivation of any required calibration functions can be accomplished by comparing rates of sap flow determined by the heat pulse method with rates measured by an independent method (gravimetrically if possible).

Cohen et al. (1981) developed an alternative improved heat pulse method that relies on measuring the time tm recorder to reach a maximum temperature in a single sensor located downstream at distance xd from the lime heater. This method is referred as the Tmax method. The heat pulse velocity Vm (ms−1) is calculated from
$${\textrm{V}}_{\textrm{m}} = ({\textrm{x}}_{\textrm{d}} ^2 - 4{\textrm{Kt}}_{\textrm{m}} /{\textrm{t}}_{\textrm{m}} )^{0.5}$$
The only other factor required to determine Vm is the thermal diffusivity K, which is determined from the following equation:
$${\textrm{K}} = {\textrm{xd}}^{\textrm{2}} /4{\textrm{t}}_{\textrm{m}}$$

That is calculated assuming zero sap flow.

Calculations of Vz and Vm assume that the heater and temperature probes have no effect on the measured heat flow. In reality, convection of the heat pulse is disturbed by the presence of the heater and the temperature probes and also by the disruption of the xylem tissue with their placement. These perturbations produce a systematic underestimation in the measured heat pulse velocity (Cohen et al. 1981, Green et al. 2003).

Recently, Intrigliolo et al. (2009), used this system for measuring sap flow in grapevines with small trunks and under cold winters that might have caused xylem injuries. They use two thermocouples at a radial distance of 5 and 12 mm from the surface of the trunk (without bark). One probe measures the maximum temperature rise at distance of 10 mm downstream from the heater. The second reference probe, which is located 40 mm below the heater measures changes in ambient stem temperature (Green et al. 2003). Heat pulse velocity (Vc) is converted into sap flow (Vs) using \({\textrm{V}}_{\textrm{s}} = {\textrm{V}}_{\textrm{c}} (0.441{\textrm{ F}}_{{\textrm{wood}}} + {\textrm{F}}_{{\textrm{water}}} )\), where Fwood and Fwater are calculated from the volume fractions of wood and water respectively (Becker and Edwards 1999). These authors estimate the portion of sapwood (tissue actively involved in transporting water in the trunk cross sectional area) at the end of the season by cutting trunk sections at the height of the gauges insertion and analysing it by binocular microscopy. The determination of volume fraction of wood and water can be made by Archimede’s principle by coring a sample from different vines in the same vineyard and assuming that no air is present in the sapwood tissue. The results of their experiments show some deviations and errors associated to the technique. Comparison of sap flux data measured by Tmax-heat pulse method with other transpirations determinations (gas exchange) showed that five out of six heat pulse gauges clearly underestimated canopy transpiration using the standard assumptions. In addition, the deviations from the actual values were considerable and varied from vine to vine. This suggests that there was not a systematic error in sap flow readings, but rather a seemingly random deviation from the actual values. Water transport in the sapwood is generally not uniform across the trunk section (Braun and Schmid 1999b). Hence, there might have been differences between vines in the location of the probes with respect to the portion of the trunk’s sapwood that transport the majority of the water (Fig. 5.2).
Fig. 5.2

Scheme and picture of HPV sensors installation (with permission of Fernández and Intrigliolo respectively)

The heat ratio method (HRM), is a heat-pulse method developed by Burguess et al. (2001), able to accurately measure low rates of sap flow. The HRM method measures the ratio of the increase in temperature, following the delivery of a heat pulse, at points equidistant downstream and upstream from the linear heater. HRM configuration, correction for wounding and other operational details are given by Burguess et al. (2001). The HRM method is sensitive to the direction of sap flow, being able to measure reverse flow in roots and other conductive organs.

5.2.2.1 Practical Considerations

It is essential to position the heater and sensor probes correctly. Accurate spacing between the probes is achieved by using a guide jig when drilling the holes, which must be done carefully to prevent excessive damage to the stem. The exact position of each probe should be carefully measured, so that errors caused by misalignment of probes can be corrected. In general, probe sets should be moved regularly to new stems because wound reactions in the woody tissue of the stem often develop 14–21 days after implanting the probes. These reactions are thought to be caused by the deposition of resin in the xylem vessels or tracheas surrounding the implantation site (conifers and other species), or possibly cavitations (vines), with the result that sap flow moves away from the sensor probes as the wound reaction develops, seriously decreasing the accuracy of the technique (Fig. 5.3).
Fig. 5.3

Damage produced by probes insertion in. sap stream section

Unusually, wound reactions can develop in much less than 14 days and so it is essential that users of the heat-pulse technique monitor their data for changes in sensitivity to sap flow and move the probes more frequently when necessary.

5.2.3 Other Invasive Methodologies

Heat field deformation (HFD). This method, developed by Nadezhdina et al. (1998), is based on the analysis of temperature differences around a linear heater inserted in the sapwood. These temperature differences characterise the deformation of the heat field around the heater caused by the ascent of sap. The HFD method can measure wide-sized stems and trunks in a broad range of flow rates including low, zero and reverse flows. The HFD sensors consist of a linear heater and two pairs of differential thermocouples (Symmetrical and asymmetrical) measuring the temperature in axial (dTsym) and tangential (dTasym) directions around the heater (Nadezhdina et al. 2002) as raw data. Sap flow is then calculated from the mentioned temperature differences. The multi-pint sensor has several thermocouples along each needle and allows measurements of sap flow radial profile. dTsym is also known as the sap flow index (SFI), which can be used as a stress indicator (Nadezhdina 1999).

The HFD system is available from ICT International (see Table 5.1)
Table 5.1

List of main companies that manufacture sap flow equipment and types of system

Manufactures of sap flow equipments

Sap flow techniques

Dynamax, Inc.

TDP sap velocity (HD)

10808 Fallstone Road Suite 350, Huston USA

www.dynamax.com; www.fruitionsciences.com

Dynagage (HB, Sap flow sensors from 2.1 to 165 mm stem diameter)

 

Flow 4 (data analysis system)

East 30 Sensors

 

1610 Kitzmiller Road Pullman, Washington USA

Sap flow sensors (HP)

support@east30sensors.com

 

Ecomatik

 

Muenchener str.22. D85221 Munich Germany

SF-L sensor (HD sap flow sensor)

www.ecomatik.de

 

EMS Brno

 

Turisticka 5 62100 Brno Czech Republic

www.emsbrno.cz

Sap flow meter T4.2 (HB external heating 6–20 mm stems and branches)

 

Sap flow EMS 51 (THB for large stems (from 12 mm)

ICT International

 

PO Box 503. Armidales NSW 2350 Australia

Sap flow systems (HFD, HRM methods)

www.ictinternational.com

 

Phytech Ltd.

 

Kibbutz Yad Mordechai, 79145 Israel

www.phytech.com

SF4 & SF5 sap flow sensors (HB from 1 to 10 mm diameter)

 

SF8 sap flow sensor (Granier type above 15 mm diameter)

Tranzflo NZ Ltd.

 

15 Parata St. Palmerston North 4410. New Zealand

The Green’s HPV system

www.tranzflow.co.nz

 

UP-GmbH

 

Bahnhofstrasse D-03046 Cottbus Germany

Ex618 M1 and BAS (HD sap flow system)

www.upgmbh.com

 

Abbreviations: HPV: heat pulse velocity; HP: heat pulse method; CHP: compensation heat pulse HD: heat dissipation; HFD: heat field deformation; HRM: heat ratio method; SHB: stem heat balance; THB: trunk heat balance

5.3 Non-invasive Methodologies

5.3.1 Stem Heat Balance Method (SHB)

Heat is applied to the entire circumference of the stem encircled by the heater and the mass flow of sap is obtained from the balance of the fluxes of heat into and out of the heated section of stem (Sakuratani 1981, Baker and van Bavel 1987). The foam insulation and weather shield surrounding the stem extend above and below the heater sufficiently to minimize extraneous thermal gradients across the heater section of stem and reduce solar heating of the stem to a negligible level. Heat input to the stem section is thus limited to the electrical power supplied to the heater (P).

The heat balance of the stem is as follows:
$${\textrm{P}} = {\textrm{Q}}_{\textrm{v}} + {\textrm{Q}}_{\textrm{r}} + {\textrm{Q}}_{\textrm{f}}$$
where Qv is the rate of vertical heat loss by conduction in the stem, Qr is radial heat loss by conduction and Qf is heat uptake by the moving sap stream. The value of Qf is determined by subtracting Qv and Qr from P. The value of P is calculated from the electrical resistance and voltage across the heater, while Qv and Qr are determined from measurements of dTA and dTB and dTr. Finally, Qf is converted to the mass flow rate of sap (Fig. 5.4).
Fig. 5.4

Scheme of heat balance method (from Dynamax Inc.)

The value of Qv is calculated using Fourier’s law for one-dimensional heat flow from the upward and downward temperature gradients away from the heater (Sakuratani 1981, Baker and van Bavel 1987). The radial component of the stem heat balance Qr directly depend on the effective thermal conductance (Ksh) of the sheath of materials surrounding the heater. The value of Ksh is unknown and depends of the thermal conductivity of the insulating sheath and stem diameter. This component should to be calculated during periods when no sap flows are known to be zero.

Once all other components of the stem heat balance are known, Qf is determined by difference and mass flow rate of sap (Fm) using:
$${\textrm{F}}_{\textrm{m}} = {\textrm{Q}}_{\textrm{f}} *{\textrm{c}}_{\textrm{s}} ^{ - 1} *{\textrm{dT}}_{{\textrm{AB}}} ^{ - 1}$$

where cs is the specific heat capacity of sap and dTAB is the sap temperature gradient across the heater assuming that heating of the sap is uniform in radius.

Stem heat balance gauge includes a flexible heater, typically a few cm in width, which is wrapped around the stem and enclosed in a layer of cork, a layer of foam insulation and aluminum coated PVC weather shield. Pairs of thermocouple junctions connected in series are embedded in the cork band to form a thermopile; one junction from each pair is positioned on the inner surface of the cork and the other on the outer surface, so that the thermopile measures the radial temperature gradient away from the heater (dTr). Gauges also contain another set of thermocouples composed of two pairs of thermocouple junctions. These thermocouples are positioned against the surface of the stem and are aligned axially along the stem, with one junction from each pair above the heater and one below, in a staggered arrangement. The two thermocouple pairs the temperature gradients dTA and dTB which are used to calculate components in the heat balance of the stem (Fig. 5.4).

5.3.1.1 Practical Considerations

Stem heat balance method can be used to measure sap flow in both woody (Steinberg et al. 1989) and herbaceous (Baker and van Bavel 1987) stems. Stem heat balance gauges to fit stems with diameter ranging from 2 to 125 mm are commercially available (see Table 5.1). Individual gauges can only be used on stems with diameter ranging within relatively narrow limits, for example 2–3.5 mm for the smallest gauges and 100–125 for the largest. As a consequence, a number of different size gauges are required when sap flow measurements must be made on stems of different sizes.

Gauges should be installed on straight sections of stem without swellings or lumps that could cause poor contact between the stem surface and the heater or thermocouples. Loose bark and any small branches or leaves sprouting from the section to be enclosed by the gauge should be carefully removed. Application of silicone-grease-based electrical insulating chemical to the stem surface prior to installation of the gauge is usually recommended for several reasons: to ensure good thermal contact between the gauge and stem, to allow slippage of the gauge during installation, to prevent ingress of water and condensation, to prevent sensor corrosion, and to allow movement of the gauge during contraction and expansion of the stem. After preparation of the surface, the gauge is positioned on the stem and fixed using Velcro straps. It is important that water entry is prevented, as it can cause erratic measurements and damage the electrical components of the gauge. This can be completely prevented by attaching a conical collar made of heavy-duty polythene to the stem just above the gauge and sealing the joint with grafting wax.

In the commercially available systems, constant power is supplied to the gauge heater. The voltage across the heater must be adjustable so that it can be set between 3 and 10 V, depending on the size of the gauge and sap flow rates. The voltage should be selected to maintain a measurable increase in sap temperature (> 1ºC) during periods of high sap flow, but without heating the stem to damaging temperatures when flow is low.

Systems with a variable power supply that is controlled to maintain a constant increment in stem temperature have also been constructed (Grime et al. 1995, Weibel and Boersma 1995), but the power control requires either complex data-logger programs or additional circuitry. Nevertheless, variable power systems have important advantages over the conventional constant power systems: overheating of the stems at low flow rates is avoided; power consumption is lower, an important consideration when operating sap flow gauges at remote sites away from mains power; the dynamic response is improved, and the heat storage term is reduced. Also, the advantage of variable power control for heat balance sap flow gauges are evident under high flow rates (> 600 g h−1), where high rates of power must be applied. Grapevine is a good example of this fact. Recently, Tarara and Fergurson (2006) have developed algorithms (open-loop algorithms) based on day length and the theoretical diurnal course of irradiance, that reduce the possible errors in Qv and Qr calculation.

Accurate determination of rates of sap flow using stem heat balance gauges critically depends on the correct evaluation of the effective thermal conductance (Ksh), which must be done when sap flow is zero. For this propose, it is often assumed that there is no sap during the hours before dawn (Steinberg et al. 1989). However, sap flow can occur at night particularity under conditions of dry air advection (Green et al. 1989), and so it can be necessary to determine Ksh in other ways, for example, from data obtained after heave rainfall, or after all foliage (Steinberg et al. 1989). A commercial equipment by Environmental Systems inc. provide very simple gauges containing a heater system with electrical resistances connected in series and two thermocouples inserted in the stem, that measure the temperature up and down from the heater. The gauges easy to install and the software permit to obtain directly the mass flow of the stem subtracting day by day the sap flow measured during the night.

Different equipments based on heat balance methodologies have been used to measure sap flow in grapevines (Eastham and Gray 1988, Lascano et al. 1992, Braun and Schmid 1999a, Calo et al. 1999, Escalona et al. 2002). In general, data obtained from heat balance gauges provide a good estimation of plant water consumption. However, the literature also provides examples of deviations between the water use measured with the stem heat balance and that determined by the transpiration method with canopy gas exchange equipments (Dragoni et al. 2006). Also, it is a very interesting tool for ecophysiological studies and agronomical applications.

5.3.2 Other Non-invasive Methods of Sap Flow Measures

Nuclear magnetic resonance has been used to measure phloem and xylem sap flow velocities non-invasively (Xia et al. 1993, Peuke et al. 2001). A pulse magnetic field gradient is applied along the direction of flow causing proton spin at frequency directly proportional to the intensity of the applied magnetic field. An inverse magnetic field gradient is applied after a certain time, turning all spins back to a net phase shift of zero. However, if the protons have moved between the respective applications of the magnetic field gradients, the phase shifts will not return to zero. These phase shifts are proportional to flow velocity (Peuke et al. 2001). Short time resolutions and precise quantitative velocity measurements can be achieved with NMR. However, the cost and dimensions of the equipment and the complexity of data processing render NMR unsuitable for field applications.

Pulsed-laser system is based to apply a heat pulse to the surface of the stem with near-infrared laser source. The heat propagation is monitored externally by means of an infrared camera. Heat pulse velocities are determined from the thermometric data and related to the more useful quantity, mass flow rate. Based on this technique, Helfter el al. (2007) developed a compact stand-alone and non-invasive system allowing for direct detection of phloem and xylem sap movements and allows for precise measurements of phloem flow velocities.

Notes

Acknowledgment

The authors wish to acknowledge Mike van Bavel (Dynamax), Diego Intrigliolo and Enrique Fernández for their contributions on this chapter as specialists in sap flow techniques and for allowing use of pictures and schemes. Funding was provided by the Spanish Ministry of Education (project AGL 2008-04525-02-01). J. Escalona benefit from Balearic Government and Balearic University.

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Copyright information

© Springer Science+Business Media B.V. 2010

Authors and Affiliations

  1. 1.Grup de Recerca en Biologia de les Plantes en Condicions Mediterrànies Universitat de les Illes BalearsPalma de MallorcaSpain
  2. 2.Departamento de Biologia, Grup de Recerca en Biologia de les Plantes en Condicions MediterràniesUniversitat de les Illes BalearsPalma de MallorcaSpain

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